Multilayer deposition method for forming Pb-doped Bi-Sr-Ca-Cu-O Superconducting films

ABSTRACT

A perovskite type superconductor film having a high content, almost a single phase, of the high Tc phase is formed by the steps of: depositing at least one first film of a first material (e.g., a composite oxide of Bi-Sr-Ca-Cu-O system or Tl-Ba-Ca-Cu-O system) constituting a perovskite type superconductor over a substrate; depositing at least one second film of a second material containing an oxide or element (Bi 2  O 3 , Tl 2  O 3 , PbO x , etc., particularly PbO x ) having a vapor pressure of more than 10  -4  Pa at 800° C. at least as a main component over the substrate; to thereby form a stack of the first and second films; and heat treating the stack of the first and second films to form the perovskite type superconductor film on the substrate. Further, preferred compositions of the as-deposited films or stack are determined.

This is a continuation of copending application Ser. No. 07/442,624,filed on Nov. 29, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a perovskitetype superconductor film, particularly Bi-(Pb)-Sr-Ca-Cu-O andTl-Ba-Ca-Cu-O systems with a high content of the high Tc phase, i.e.,having a high critical temperature Tc of superconductivity.

2. Description of The Related Art

The research and development of high temperature superconductivematerials exhibiting superconductivity at above the boiling point ofliquid nitrogen is proceeding rapidly, and investigations into practicalapplications thereof are also underway. Particularly, superconductorshaving a critical temperature Tc above 100K are under investigationbecause they advantageously have a large temperature margin for liquidnitrogen application, theoretically have an increased critical currentdensity Jc and critical magnetic field Hc.

Known superconductors having a critical temperature Tc above 100K areBi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. These Bi- and Tl- systems havethe advantages of a resistance to water and oxygen degradation.Nevertheless, the Bi- and Tl-system superconductors have a disadvantagein that a low Tc phase is easily formed and the formation of single highTc phase is difficult. The Bi-system is typically a mixed phase of a lowTc phase with a critical temperature Tc of about 80K and a high Tc phasewith a Tc of about 110K. The Tl-system is typically a mixed phase of alow Tc phase with a critical temperature Tc of about 105K and a high Tcphase with a Tc of about 125K. The formation of isolated high Tc phasehas not been accomplished.

A new high Tc Bi-Sr-Ca-Cu-O system superconductor was found by Maeda etal (Jpn. J. Appl. Phys. 27, 1988, L209). It was also found that thissystem contains three superconducting phases represented by the formulaBi₂ Sr₂ Ca_(n-1) Cu_(n) O_(x). A first phase corresponding to n=1 in theformula and having a Tc of 7K, a second phase corresponding to n=2 andhaving a Tc of 80K, and a third phase corresponding to n=3 and having aTc of 105K. With the increase of n from 1 to 3, the number of CuO layersin the crystal structure is increased from 1 to 3 and the c axis of thecrystal is elongated from 2.4 nm to 3.0 nm to 3.7 nm.

Takano et al reported that the volume fraction of the high Tc phase isincreased by partially substituting Pb for Bi (Jpn. J. Appl. Phys. 27,1988, L1041), but the role of Pb is not clear at present.

In a typical process for preparing a superconductor film of, forexample, a Bi-Sr-Ca-Cu-O system superconductor, an oxide is deposited ona substrate of MgO, SrTiO₃, etc. by sputtering, evaporation, etc.,followed by a heat treatment to react the deposited film, to therebyform a superconducting film. There is a need to provide a process forreproduciblly forming a single phase film, having a high Tc phase, ofthe Bi-(Pb)-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems.

SUMMARY OF THE INVENTION

The present invention provides a process for preparing a perovskite typesuperconducting film, comprising the steps of: depositing at least onefirst film of a first material constituting a perovskite typesuperconductor over a substrate; depositing at least one second film ofa second material containing an oxide having a vapor pressure of morethan 10⁻⁴ Pa at 800° C. at least as a main component over the substrate,thereby forming a stack of the first and second films; and heat treatingthe stack of the first and second films to form the perovskite typesuperconductor film on the substrate.

Particularly, the present invention provides a process for preparing aperovskite type superconductor film on a substrate, comprising the stepsof: depositing a composite oxide film of Bi-Sr-Ca-Cu-O system having athickness of 50 to 2,000 nm on a substrate; optionally depositing a Bi₂O₃ film having a thickness of 5 to 20 nm on the first composite oxidefilm at a first temperature of 200° to 500° C.; depositing a PbO filmhaving a thickness of 5 to 20 nm on the first composite oxide film at asecond temperature less than 500° C.; optionally depositing a CuO filmhaving a thickness of 5 to 30 nm at a third temperature of 200° to 500°C.; repeating the deposition of the composite oxide, Bi₂ O₃ if necessaryand PbO films to form a stack of the composite oxide film, the Bi₂ O₃film if present, the CuO film if present, and the PbO film on thesubstrate, the stack having a top film of the composite oxide film; andheat treating the stack at a third temperature of 835° to 870° C. higherthan the first and second temperature to form a film of aBi-(Pb)-Sr-Ca-Cu-O perovskite type superconductor on the substrate.

The present invention also provides a process for preparing a perovskitetype superconductor film on a substrate, comprising the steps of:depositing a composite oxide film of Tl-Ba-Ca-Cu-O system having athickness of 50 to 2,000 nm on a substrate; depositing a Tl₂ O₃ filmhaving a thickness of 10 to 20 nm on the first composite oxide film at afirst temperature of 200° to 500° C.; repeating the deposition of thecomposite oxide and Tl₂ O₃ films to form a stack of the composite oxideand Tl₂ O₃ films on the substrate, the stock having a top film of thecomposite oxide film; and heat treating the stack at a secondtemperature of 750° to 890° C. higher than the first temperature to forma film of a Tl-Ba-Ca-Cu-O perovskite type superconductor on thesubstrate.

By the present invention there is also provided an optimum compositionfor forming high Tc phase Bi₂ Sr₂ Ca₂ Cu₃ O_(x) in Pb-doped Bi systemthin films. Although it was reported that doping with Pb enhanced thehigh Tc phase formation in the bulk system, the present inventors foundthat in thin films, doped Pb easily evaporates during post-annealing orsintering and a larger amount of Pb is necessary to synthesize the highTc phase. Further, the present inventors optimized the Cu content in thedeposited film. A slightly excess Cu effectively decreases the low Tcphase.

Thus, a preferred composition of the as-deposited film beforepost-annealing is a ratio of Bi/Pb of 1/0.5-1.5, more preferably1/0.6-0.9, and a ratio of Sr/Cu of 1.5-1.7. Also a most preferredcomposition is Bi:Pb:Sr:Ca:Cu of 0.8-1.1:0.5-1.0:1:0.9-1.1:1.5-1.7.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show X-ray diffraction patterns of Bi-Sr-Ca-Cu-O andBi-Pb-Sr-Ca-Cu-O superconductor films in the prior art;

FIG. 3 shows the electric resistivity v.s. the temperature of aBi-Pb-Sr-Ca-Cu-O superconductor film in the prior art;

FIG. 4 shows a structure of an as-deposited stack of the presentinvention;

FIG. 5 shows the electric resistivity v.s. the temperature of aBi-Pb-Sr-Ca-Cu-O superconductor film of the present invention;

FIG. 6 shows the electric resistivity v.s. the temperature of aBi-Pb-Sr-Ca-Cu-O superconductor film of the present invention at severalthe current densities;

FIG. 7 shows a structure of another as-deposited stack of the presentinvention;

FIG. 8 shows X-ray diffraction patterns of Bi-Pb-Sr-Ca-Cu-Osuperconductor films of the present invention;

FIG. 9 shows the composition of a Bi-Pb-Sr-Ca-Cu-O system superconductorfilm during a heat treatment;

FIG. 10 shows the electric resistivity v.s. the temperature of aBi-Pb-Sr-Ca-Cu-O superconductor films of the present invention;

FIG. 11 shows X-ray diffraction patterns of Bi-Pb-Sr-Ca-Cu-Osuperconductor films of the prior art;

FIG. 12 shows the electric resistance v.s. the temperature, ofBi-Pb-Sr-Ca-Cu-O superconductor films of the prior art;

FIG. 13 shows a structure of a further as-deposited stack of the presentinvention;

FIG. 14 shows an X-ray diffraction pattern of a Bi-Pb-Sr-Ca-Cu-O systemsuperconductor film of the present invention;

FIGS. 15A and 15B show microstructures of a Bi-Pb-Sr-Ca-Cu-O systemsuperconductor film of the present invention;

FIG. 16 shows the electric resistance v.s. the temperature of aBi-Pb-Sr-Ca-Cu-O system superconductor film of the present invention;and

FIG. 17 shows X-ray intensity ratios of high Tc/low Tc at several Cu/Srand Pb/Bi ratios.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O system superconductors comprise at leastfour metal elements, and as a result, it is difficult to adjust thecomposition to a desired value. Further, metal elements having arelatively high vapor pressure, such as Bi, Pb and Tl, tend to be lostfrom a deposited film, e.g., by evaporation during the deposition andannealing of the film. The amounts of the loss of the elements in thedeposited film are changed by the substrate temperature, the depositionrate of the film and the annealing temperature. Therefore, thecomposition of the film is not reproducible.

The vapor pressure (P) of an oxide is represented by the formula: logP=AT⁻¹ +B logT+C×10⁻³ ×T+ D where T stands for the absolute temperature(K), and A, B, C and D are constants ("Metal Data Handbook", p 86).Examples of the vapor pressure of elements and oxides at 1073K (800° C.)are shown below:

    ______________________________________                                        Element                log P    P.sub.0                                       Oxide A        B        C     D    (mm Hg)                                                                              (Pa)                                ______________________________________                                        PbO   -13480   -0.92    -0.35 14.36                                                                              -1.366 0.057                                                                  -8.507                                                                        at                                                                            400° C.                             Pb    -10130    -0.985  11.16      -1.266 0.072                               Bi    -10400   -1.26          12.35                                                                              -1.16  0.092                               Cu    -17700   -0.86    12.29      -6.812 2 × 10.sup.-7                 ______________________________________                                    

FIG. 1 shows a X-ray diffraction pattern of a Bi-Sr-Ca-Cu-O film formedin accordance with the prior art. A sintered oxide target of Bi₂ Sr₂ Ca₂Cu₃ O_(x) was prepared by firing a mixture of oxides in air at 800° C.for 24 hours. A film was deposited by RF magnetron sputtering on a MgOsubstrate from the above target heated to 400° C. under 1 Pa, and theobtained film having a thickness of 1 μm was then heat at 875° C. for 5hours. The thus-obtained film compromised of, as seen from FIG. 1, Bi₂Sr₂ CuO_(x) and Bi₂ Sr₂ CaCu₂ O_(x) (the low Tc phase having a Tc of80K). The film had a composition of Bi:Sr:Ca:Cu=0.63:1.00:1.07:1.40 byEPMA (electron probe micro analysis), which was greatly deviated fromthe stoichiometric composition of the high Tc phase (Bi₂ Sr₂ Ca₂ Cu₃O_(x)) having a Tc of 110K.

When Pb was added to the above target, so that the Pb-doped target had acomposition of Bi₂ Pb₀.4 Sr₂ Ca₂ Cu₃ O_(x), and the deposited film washeat treated at 850° C. for 12 hours, the resultant film contained ahigh Tc phase. However, the volume fraction was small (see FIG. 2). FIG.3 shows the electric resistantivity of the film vs. the temperature, inwhich the on-set is seen to be around 110K but zero resistance is notachieved until 75K. It was when an appropriate amount of Pb is added toa bulk sample of Bi-Sr-Ca-Cu-O, a high volume fraction of the high Tcphase is obtainable, by Koyama et al (JJAPL. vol. 27, pp-L1861-L1863), asufficient amount of Pb was not added to the deposited film because Pbis easily evaporated during the deposition and annealing of the film.

The present invention resolves the above problem by independentlyforming a layer containing, at least as a main component, an oxide orelement having a high vapor pressure of more than 10⁻⁴ Pa, particularlymore than 10⁻², at 800° C., such as Bi₂ O₃, PbO_(x) or Tl₂ O₃, in astack of oxide layers, to have an average composition close to a desiredvalue whereby, after heat treating the stack for a short time, asuperconductor film comprising a phase having a structure containingthree CuO planes in a 1/2 unit, the superconductor film having a highcritical temperature and a high critical current density, is thusobtained.

Thus, the present invention provides a process for preparing aBi-(Pb)-Sr-Ca-Cu-O or Tl-Ba-Ca-Cu-O system superconductor film,comprising the steps of: depositing at least one first film of a firstoxide constituting a perovskite type superconductor over a substrate ata first temperature; depositing at least one second film containing asecond oxide having a vapor pressure of more than 10⁻⁴ Pa at 800° C. atleast as a main component of the second film over the substrate, therebyforming a stack of the first and second oxide films; and heat treatingthe stack of the first and second oxide films at a second temperaturehigher than the first temperature in an atmosphere containing oxygen toform the perovskite type superconductor film on the substrate.

At least one oxide having a vapor pressure of more than 10⁻⁴ Pa at 800°C. is independently deposited because, at said first temperature forsaid deposition if such a high vapor pressure oxide is deposited at thistemperature, and at said second temperature for said heat treatment,said oxide or the element of said oxide is easily evaporated and thecomposition of the film is remarkably varied, and therefore acompensation of the evaporating component is necessary, in order toobtain a high Tc phase superconductor with the stoichiometriccomposition.

The process described herein may be applied to other supper conductors.Important examples thereof are Bi₂ (Sr_(1-x) Ca_(x))_(m) Cu_(n) O_(z),where 0<x<1, m=3 and n=2, or m=4 and n=3, and z>0; and Tl₁ (Ba_(1-x)Ca_(x))_(m) Cu_(n) O_(z) or Tl₂ (Ba_(1-x) Ca_(x))_(m) Cu_(n) O_(z),where 0<x<1, m=3 and n =2, or m=4 and n=3, and z>0, or a composite ofthese last two. In these superconductors, it is considered that the lowTc phase has a composition close to m=3 and n=2 and the high Tc phasehas a composition close to m=4 and n=3. It should be noted that theaverage composition of the high Tc phase superconductor may show somevariation (at largest 10%) from the stoichiometric values. Theperovskite type superconductor of the present invention also includesthe superconductor having the above composition, Bi₂ (Sr_(1-x)Ca_(x))_(m) Cu_(n) O_(z), in which a part of an element or elementsthereof is replaced by Pb and/or Pb is further added. Suchsuperconductors may be represented by the formula: Bi_(2-y) Pb_(r)(Sr_(1-x) Ca_(x))₄ Cu₃ O_(z) where 0<x<1, y>0, r>0, an z>0. Also, in theabove Tl-system, a part of an element or elements thereof may bereplaced by Pb.

The oxides or elements having a vapor pressure of higher than 10⁻⁴ Pa at800° C. in the above superconductors are Bi₂ O₃ (Bi), PbO_(x) (Pb), andTl₂ O₃ (Tl). These oxides are preferably deposited at a temperature (forexample, less than 500° C.) lower than a temperature (typically750°-890° C.) of a heat treatment for forming a perovskite typesuperconductor. A preferable temperature for depositing the oxides,particularly PbO, is 200° to 500° C., more preferably 350° to 400° C. Ata temperature higher than 500° C., evaporation occurs, and at atemperature lower than 200° C., the adhesion of the deposited oxide filmis poor.

In another aspect of the present invention, the film ofBi-(Pb)-Sr-Ca-Cu-O system as deposited and before the heat treatmentshould have an average composition with a ratio of Bi/Pb of 1/0.5-1.5,more preferably 1/0.6-0.9. In the prior art ratio of Bi/Pb of about1.8/0.34 (i.e., 1/0.189) is thought preferable as a starting materialfor forming a bulk superconductor (Kawai et al, Jpn. J. appl. Phys. 27,1988, L1476). Nevertheless, the present inventors found that Pb orPbO_(x) in a film is preferentially and greatly evaporated during a heattreatment of the stack, and the high Tc phase is formed mainly when Pbremains in the film after deposition, and therefore, to form a high Tcphase superconductor film, Pb is necessary in an as-deposited film in anamount more than the amount adequate for bulk samples. But if too muchPb is in the as-deposited film, the film is partially fused during theheat treatment and the heat treated film phase separated.

In addition to Pb doping, it is found that an Sr/Cu ratio of 1/1.5-1.7is desired. In this case, the most preferred composition of theas-deposited film can be expressed by a ratio of Bi:Pb:Sr:Ca:Cu=0.8-1.1:0.5-1.0:1:0.9-1.1:1.5-1.7. An excess amount of CuO is preferredbecause the high Tc phase is more easily formed when an excess amount ofCuO is present.

CuO may be deposited as a separate CuO film in a stack of films, as theabove described element or oxide having a high vapor pressure.

The deposition of the oxides may be performed by physical vapordeposition such as sputtering, evaporation, molecular beam epitaxy(MBE), etc., or chemical vapor deposition (CVD) including plasma CVD,etc., or other processes.

Certain elements or oxides of the superconductor may be depositedtogether because they do not have a high vapor pressure, and therefore,can be deposited without hard evaporation even by simultaneousdeposition, or if desired may be separately deposited, particularly CuO.

The thickness of the film having a low vapor pressure of the oxide orelement is 50 nm to 200 nm.

The order of the deposition of the films is preferably: first depositingan oxide having a low vapor pressure directly on a substrate so thatadhesion of the film to the substrate is firm; and last depositing anoxide having a low vapor pressure as the top film of the stack toprevent evaporation of oxide or element having a high vapor pressurefrom the top of the stack during the heat treatment in order to form theperovskite type superconductor.

The stack of the films may have a composition which is stepwise orgradually changed from one film to an adjacent film.

The number of films in the stack is not limited, and a combination of afilm of an oxide having a high vapor pressure and a film of an oxidehaving a low vapor pressure may be repeatedly stacked.

The stack thus obtained is heat treated at a temperature higher than thetemperature at which the oxide was deposited. By this heat treatment, aperovskite type superconductor is formed having an approximatelystoichiometric composition and containing the high Tc phase at a highvolume fraction. After the heat treatment, the amount of the oxidehaving a high vapor pressure may be lower than that of the as-depositedstack. Particularly the content of Pb is often reduced or evencompletely lost after the heat treatment, but a high volume fraction ofthe high Tc phase is obtainable. This suggests that Pb acts as anaccelerator for forming the high Tc phase and is not necessarily acomponent of the high Tc phase. A separate formation of the PbO film isan important feature of the present invention. A preferred content ofthe PbO in the as-deposited stack has been described.

The temperature of the heat treatment is 835° C. to 870° C. At atemperature lower than 835° C., a low Tc phase superconductor is alsoformed, and at a temperature higher than 870° C., the superconductor maybe melted.

In accordance with the present invention, among elements constituting aperovskite type superconductor, an oxide of an element having a highvapor pressure and difficult to deposit or an element or oxide which isrelatively easily evaporated during a post-annealing of the depositedfilm is independently and concentratedly deposited, and thus the timefor depositing such an oxide is shortened and variations of thecomposition by evaporation during the deposition and the annealing ofthe deposited film or stack can be reduced or neglected. By depositingan oxide with a low vapor pressure immediately on the above oxide,evaporation of the stack as a whole is also largely prevented. Thus, inaccordance with the present invention, the composition of the film isvery precisely controlled and a perovskite type superconductor having ahigh Tc and a high Jc is finally obtained.

Also, in accordance with the present invention, particularly by addingto an as-deposited film or stack an appropriate amount of Pb and aslightly excess amount of CuO, Bi-(Pb)-Sr-Ca-Cu-O system, superconductorwhich is almost a single phase of the high Tc phase, and therefore, hasa high Tc and a high current density, is obtained after a heat treatmentor annealing. This is a most important feature and effect of the presentinvention.

EXAMPLES

In the following Working Examples 1-2 and Comparative Examples 1-2,depositions were carried out by a RF magnetron sputtering under thefollowing conditions:

Substrate: Single crystal (100) MgO;

Temperature of substrate: 400° C.;

Atmosphere: A mixed gas of Ar and O₂ with Ar:O₂ =1:1;

Pressure: 1 Pa;

RF power: 100 W (about 1.3 W/cm²);

Target A: Bi₂ Sr₂ Ca₂ Cu₃ O_(z) sintered at 800° C. for 24 hours;

Target B: Bi₂ Pb₀.4 Sr₂ Ca₂ Cu₃ O_(z) sintered at 800° C. for 24 hours;

Target C: Bi₂ O₃ ;

Target D: PbO_(x).

COMPARATIVE EXAMPLE 1

A film was deposited from the target A (Bi:Sr:Ca:Cu =2:2:2:3) onto a MgOsubstrate heated at 400° C. to a thickness of 1 μm, and then heated inair at 875° C. for 5 hours.

FIG. 1 shows the X-ray diffraction pattern of the heat-treated sample.It is seen from FIG. 1 that the phase of Bi₂ Sr₂ CuO_(x) having a Tc of10K and the low Tc phase of Bi₂ Sr₂ CaCu₂ O_(x) having a Tc of 80K wereformed.

The composition of the film determined by EPMA wasBi:Sr:Ca:Cu=0.63:1.00:1.07:1.40, in which the content of Bi wasremarkably decreased from the composition of the target ofBi:Sr:Ca:Cu=2:2:2:3.

COMPARATIVE EXAMPLE 2

A film was deposited from the target B (Bi:Pb:Sr:Ca:Cu=2:0.4:2:2:3) ontoa MgO substrate heated at 400° C. to a thickness of 1 μm, and thenheated in air at 850° C. for 12 hours.

FIG. 2 shows the X-ray diffraction pattern of the above heat treatedsample. It is seen from FIG. 2 that the high Tc phase of Bi₂ Sr₂ Ca₂ Cu₃O_(z) was formed but the volume fraction thereof was still very low incomparison with the volume fraction of the low Tc phase of Bi₂ Sr₂ CaCu₂O_(x).

FIG. 3 shows the electric resistivity dependence on the temperature inwhich the on-set temperature was higher than 110K but the Tce (end pointof the critical temperature) was 75K.

The composition of the film was determined by EPMA and it was found thatBi was lacking and the composition was non-stoichiometric, and that onlya little Pb was doped in the deposited film (therefore, it is understoodthat the content of Pb was not enough to form a high Tc phase).

WORKING EXAMPLE 1

A film was deposited from the target A (Bi:Sr:Ca:Cu =2:2:2:3) and thetarget C (Bi₂ O₃) onto a MgO substrate heated at 400° C. to form a stackof five films of Bi-Sr-Ca-Cu-O, 200 nm thick, and three films of Bi₂ O₃,15 nm thick, the stack having a total thickness of 1 μm, and then heatedin air at 875° C. for 5 hours.

The EPMA analysis revealed that the annealed film had a composition veryclose to the stoichiometric composition of the high Tc phase, i.e., thecontent of Bi was adequately compensated.

Nevertheless, it was also revealed that the high Tc phase was formedonly in a very little amount such that it could be only just detected.

WORKING EXAMPLE 2

FIG. 4 illustrates a layer structure of the stack formed in thisexample, in which a plurality of a Bi-Sr-Ca-Cu-O film 2, a Bi₂ O₃ film 3and a PbO film 4, in this order, are deposited on a MgO substrate 1 andanother Bi-Sr-Ca-Cu-O film 2' is finally deposited on the top of thestack of the films 2, 3 and 4.

The above deposition was carried out by magnetron sputtering using thetargets A, C and D. The thicknesses of the Bi-Sr-Ca-Cu-O film 2, the Bi₂O₃ film 3, and the PbO film 4 were 300 nm, 20 nm and 10 nm,respectively, and the total thickness of the stack was about 1 μm. Thethus-obtained stack was heat treated in air at 850° C. for 10 minutes to15 hours.

The composition of the film after 1 hour heat treatment at 850° C. wasBi:Pb:Sr:Ca:Cu=0.87:0.34:1.00:1.27:1.88, (or 1:0.39:1.15:1.46:2.16), byEPMA. This clearly suggests that the amount of Bi was adequate and asufficient amount of Pb was doped in the film.

It was found that, in this example, the Pb continuously evaporatedduring the heat treatment of the stack and only a little amount of Pbremained after 3 hours at 850° C. The as-deposited oxide films wereamorphous, but reacted to form the high Tc phase after the heattreatment at 850° C. for 10 minutes, the volume fraction of the high Tcphase in the reacted film being one third. After 1 hour, more than halfof the film was high Tc phase, and after 15 hours, only a little amountof Pb remained but the film was almost entirely high Tc phase only. Thissuggests that Pb accelerates the formation of the high Tc phase but theformation of the high Tc phase does not always require Pb.

FIG. 5 shows the electric resistivity v.s. the temperature of thethus-obtained film. As seen in FIG. 5, the electric resistivity islinearly decreased with a lowering of the temperature, and rapidlydecreased at around 110K. The Tce of the film was 94.5K after 10 minutesheat treatment at 850° C. and 106.5K after 1 hour heat treatment. After15 hours heat treatment at 850° C., the Tce of the film was 105.4K andalmost the same as that after 1 hour heat treatment, but the on-settemperature was slightly elevated.

FIG. 6 shows the temperature dependence of the electric resistivity inrelation to the current density. It is seen that a satisfactorycharacteristic was obtained 2.47×10³ A/cm² at the liquid nitrogentemperature.

As understood from the above, in the process of the prior art, magnetronsputtering was carried out by using a single target (Bi₂ Sr₂ Ca₂ Cu₃ O₂)and the resultant film then heat-treated in an oxide-containingatmosphere at a certain temperature, and the resultant film has anonstoichiometric composition due to a lack of Bi. When a single target(Bi₂ Pb₀.4 Sr₂ Ca₂ Cu₃ O_(z)) doped with Pb, which is said to acceleratethe formation of the high Tc phase, is used in a similar process, theresultant film also has a nonstoichiometric composition due to a lack ofBi, and an amount of Pb sufficient to form the high Tc phase is notdeposited. As a result, the film obtained using a single target in theprior art is multiphase and the has a very small volume fraction of thehigh Tc phase.

In contrast, in accordance with the present invention, a plurality oftargets including a sintered target (Bi₂ Sr₂ Ca₂ Cu₃ O_(z), Bi₂ Pb₀.4Sr₂ Ca₂ Cu₃ O_(z), etc.) for depositing a complex oxide containing theelements constituting the perovskite type superconductor; a compensatingtarget (Bi₂ O₃, Tl₂ O₃, etc.) for depositing a compensating film tocompensate for a component which, among the elements constituting thesuperconductor, has a high vapor pressure at a temperature of the filmdeposition or at a temperature of the heat treatment or annealing, andevaporates during the steps such that the content thereof is very low inthe deposited or heat treated film; and a target of a high Tc phaseformation accelerating component (PbO_(x), etc.) for supplying a high Tcphase formation accelerating component such as PbO_(x), which is not anessential component of the high Tc phase of the superconductor and hasan effect of accelerating the formation of the high Tc phase but easilyevaporates at the deposition or heat treatment temperature as above;etc., are used to form a stack of a plurality of films on a substrateand then heat treated or annealed. This process of the present inventionobtains the following effects.

1) A component such as Bi and Tl can be adequately compensated, so thatthe composition of the formed superconductor film can be stoichiometric.

2) A sufficient amount of Pb for accelerating the formation of the highTc phase can be doped.

3) Due to the composition of the superconductor film close to that ofthe high Tc phase and a sufficient amount of Pb doped, as attained inthe above 2) and 3), a superconductor film exhibiting a high qualityhigh Tc phase can be formed by an extremely short time heat treatment.

WORKING EXAMPLE 3

A stack of films having a structure as shown in FIG. 7 was formed on asingle crystal MgO substrate by a magnetron sputtering. In FIG. 7, 11denotes the substrate, 12 a Bi-Sr-Ca-Cu-O system film having a thicknessof 200 nm, and 13 a PbO film having a thickness of 20 nm. There werefour films 12 and three films 13. The following conditions were used inthe magnetron sputtering:

Target E: a complex oxide with a ratio of Bi Sr:Ca:Cu=3:2:2:3

Atmosphere: a mixed gas of Ar and O₂ with a ratio of Ar/O₂ of 2/1

Pressure: 1 Pa

RF power: 100 W for the Bi-Sr-Ca-Cu-O film and 75 W for the PbO_(x) film

Temperature of substrate: 400° C.

The thus-obtained stack or film was composition evaluated by an ICP(induced coupled plasma) and found to have a ratio ofBi:Pb:Sr:Ca:Cu=0.9:0.8:1.0:1.2:1.7 (or 1:0.89:1.11:1.33:1.89). Theamount of Bi was adequate due to a high content thereof in the target.The ratio between Pb and Bi was 0.9:1.0, which is almost adequate.

The stack was then heat-treated in air at 850° C. for 10 minutes.

Similarly, the stacks were formed as above and then heat treated in airat 850° C. for 1 hour and 15 hours.

The thus obtained superconductor films X, Y, Z were analyzed by an X-raydiffraction and the X-ray diffraction patterns X, Y, Z are shown in FIG.8, in which H denotes the high Tc phase having the Tc of 110K and Ldenotes the low Tc phase having the Tc of 80K. In all of thesuperconductor films heat-treated for different time periods, it isclear that a large high Tc phase was formed.

FIG. 9 shows the composition of the superconductor film in relation tothe heat treatment time, evaluated by EPMA. Along with the time of theheat treatment, Pb is rapidly evaporated and disappears from the film inabout 3 hours. On the other hand, the X-ray diffraction patternsindicates that the high Tc phase was rapidly formed at the beginning ofthe heat treatment, i.e., when Pb existed. In FIG. 9, the left sideordinate represents the composition ratio and the right side ordinaterepresents the X-ray diffraction intensity ratio of the high Tc phasepeak (002) to the low Tc phase peak (002).

FIG. 10 shows the electric resistivities of the superconductor films X,Y, Z in relation to the temperature. The highest Tce was 106.5K after a1 hour heat treatment at 850° C.

COMPARATIVE EXAMPLE 3

The procedures of Working Example 3 were repeated except that thedeposited stack had a ratio of Bi/Pb of 1/0.2 by reducing the thicknessof the PbO film 13 from 30 nm to 6 nm.

The following is the change of the composition occurring during the heattreatment, as determined by EPMA.

    ______________________________________                                        Sample     Time of heat                                                       No.        treatment  Bi:Pb:Sr:Ca:Cu                                          ______________________________________                                        1          10    minutes  1.02:0.21:1.00:1.08:2.26                            2          1     hour     1.00:0.14:1.00:1.02:1.76                            3          15    hours    0.91:0.10:1.00:1.09:1.55                            ______________________________________                                    

FIG. 11 shows the X-ray diffraction patterns of the above threesuperconductors. It is clear that the amount of the high Tc phase wasnot increased by the prolongation of the heat treatment.

FIG. 12 shows the electric resistivity of the superconductor film inrelation to the temperature. The on-set temperature is 110K but the endpoint Tce is 77K.

WORKING EXAMPLE 4

A stack of films having a structure as shown in FIG. 13 was formed on asingle crystal MgO substrate by magnetron sputtering. In FIG. 13, 21denotes the substrate, 22 a Bi-Sr-Ca-Cu-O system film having a thicknessof 200 nm, 23 a PbO_(x) film having a thickness of 30 nm and 24 a CuOfilm having a thickness of 30 nm. There were four films 22 and threefilms 23 and 24. The following conditions were used in the magnetronsputtering:

Target E: a complex oxide with a ratio of Bi:Sr:Ca:Cu=3:2:2:3

Atmosphere: a mixed gas of Ar and O₂ with a ratio of Ar/O₂ of 2/1

Pressure: 1 Pa

RF power: 100 W for the Bi-Sr-Ca-Cu-O film and 75 W for the PbO_(x) filmand CuO film

Temperature of substrate: 400° C.

The composition of the thus-obtained stack or film was evaluated by anICP (induced coupled plasma) and found to have a ratio ofBi:Pb:Sr:Ca:Cu=1.0:0.8:1.0:1.0:1.6. The amount of Bi was adequate (justthe stoichiometric ratio to Sr).

The obtained stack was then heat treated in air at 850° C. for 1 hourand a superconductor film was obtained. FIG. 14 shows the X-raydiffraction pattern of the superconductor film. It is seen that analmost single phase of the high Tc phase of the superconductor wasobtained.

FIGS. 15A and 15B show microstructures of the superconductor filmobserved by a scanning electron microscope (SEM), in which c-axisaligned scaly superconductor crystals are seen.

FIG. 16 shows the electric resistivity of the superconductor film inrelation of the temperature. The resistance was rapidly lowered fromaround 106.5K and reached zero resistivity at 106.5K.

The critical current density was high, 5.8× 10⁴ A/cm² at 77.3K.

WORKING EXAMPLE 5

Superconducting films were deposited by RF magnetron sputtering withthree targets. The films were deposited on 20×20 mm² MgO (100) singlecrystals. We used a Bi-Sr-Ca-Cu-O (BSCCO) target with the composition ofBi:Sr:Ca:Cu=3:2:2:3. The film Bi content is usually less than the targetvalue, and a Bi-rich target is usually used to compensate for the lackof Bi in the films. PbO and CuO targets were also used to dope enough Pbinto the films and to optimize the Cu content. PbO and CuO layers werestacked repeatedly on BSCCO layers to precisely control composition. ThePbO and CuO layers were several dozen nanometers thick. The totalthickness of the deposited films was about 0.85 μm.

As-deposited films were amorphous and insulating. A film was dividedinto about 10×10 mm² samples, and one of these was analyzed byinductively coupled plasma analysis (ICP) to determine the filmcomposition. The other films were sintered in air for 1 h around 850° C.A muffle furnace was used and the temperature was measured by an R-type(Pt-13% Rh, Pt) thermocouple. The heating rate was 10°/min to 800° C.and 1°/min above that to avoid overshooting the set temperature. Duringheating, the films were kept at 800° C. for 20 min. The films werecooled at 10°/min. The films were examined using X-ray diffraction witha Cu Kα source and a scanning electron microscope. The electricalresistivity of the films was also measured with the four-point probemethod using dc current.

We doped the films heavily with Pb because it promoted the high Tcphase. As-deposited film compositions are listed in the following Table.Number 134 was Pb doped to the stoichiometric film at a ratio of 1.03 tothe Sr content. X-ray diffraction showed that after 1 h of sintering at852° C., large amounts of the film transformed to the high Tc phase, butabout one-fourth of the film remained in the low Tc phase. In additionto superconducting crystals, needle-like precipitations several dozenmicrons long were observed with a scanning electron microscope (SEM).Heavy Pb doping deteriorates the film morphology because the film meltsmore easily during sintering as the Pb content increases. This causes aninhomogeneous distribution of the elements and forms needle-likecrystals which were found to be Ca-Cu-O by electron probe microanalysis(EPMA) and led to a low critical current density (Jc). We found that inthin films the Cu content decreased by about 3% during an hour ofsintering.

We then attempted to decrease the amount of Pb doping and investigatedthe Cu composition dependence of the high Tc phase formation. FIG. 17shows the intensity ratios of the peaks from the high Tc phase (0014)and the low Tc phase (0012). Similar results are obtained from theratios of H(002)/L(002) and H(0010)/L(008).

                  TABLE                                                           ______________________________________                                        Atomic compositions of Bi, Pb, Sr, Ca, and Cu determined                      by ICP, normalized by the Sr composition. For the upper                       samples, PbO target was sputtered for 8 min and for the                       lower, 6 min. BSCCO target was sputtered for 100 min and                      CuO from 15 to 24 min.                                                        Run        Compositions                                                       No.        Bi:Pb:Sr:Ca:Cu                                                     ______________________________________                                        128        0.90:1.19:1.00:0.98:1.35                                           132        0.93:1.10:1.00:1.02:1.36                                           134        0.96:1.03:1.00:1.09:1.47                                           136        0.99:1.11:1.00:1.04:1.72                                           138        0.95:1.04:1.00:0.98:1.73                                           139        0.95:1.09:1.00:1.05:1.78                                           145        0.93:0.78:1.00:0.96:1.44                                           146        0.96:0.79:1.00:0.98:1.60                                           147        1.01:0.85:1.00:0.99:1.62                                           142        0.98:0.82:1.00:0.96:1.63                                           144        1.02:0.80:1.00:0.99:1.64                                           137        1.00:0.84:1.00:1.01:1.69                                           140        1.03:0.89:1.00:1.02:1.82                                           ______________________________________                                    

In FIG. 17, the large number indicate run number and the small numbersindicate sintering temperature. Dotted line is a guide for the eye. Theamount of the high Tc phase depends strongly on the Cu content. Thesintering temperature also greatly affects the amount of the high Tcphase formed. The low Tc phase forms below 848° C. and above 853° C.This suggests that the single-phase high Tc film forms in a very narrowtemperature range. For Bi:Pb:Sr:Ca:Cu=1.00:0.80:1.00:0.99:1.64, weobtained a nearly single-phase high Tc thin film after an hour ofsintering at 851° C. in air. The resistivity decreased linearly withtemperature and had a zero resistance at 106.5K. Jc reached 4.1×10⁴A/cm² at 77.3K with a criterion of 1 μV/cm.

We claim:
 1. A process for preparing a perovskite type superconductorfilm on a substrate, comprising the steps of:depositing a compositeoxide film of a Bi-Sr-Ca-Cu-O system having a thickness of 50 to 2,000nm on a substrate at a first temperature; depositing a PbO_(x) filmhaving a thickness of 5 to 30 nm on the first composite oxide film at asecond temperature less than 500° C.; repeating depositing of thecomposite oxide and PbO_(x) films to form a stack of the composite oxideand PBO_(x) films on the substrate, the stack of films as depositedhaving a top film of the composite oxide film and having an averagecomposition having a ratio of Bi:Pb:Sr:Ca:Cu of0.8-1.1:0.5-1.0:1:00.9-1.1:1.5-1.7; and heat treating the stack of filmsat a third temperature of 835° to 870° C. in an oxygen containingatmosphere to form a superconductor film, the superconductor beingrepresented by the formula Bi_(2-y) Pb_(r) (Sr_(1-x) Ca_(x))₄ Cu₃ O_(z)where 0<x<1, 2>y>0, r>0, and z>0.
 2. A process according to claim 1,wherein the second temperature is 350° to 400° C.
 3. A process accordingto claim 1, wherein the composite oxide film of the Bi-Sr-Ca-Cu-O systemof the composite oxide film is represented by the formula Bi₂ (Sr_(1-x)Ca_(x))_(m) Cu_(n) O_(z), where 0<x<1, m =3 and n=2, or m=4 and n=3, andz>0.
 4. A process according to claim 1, wherein the composite oxide filmof the Bi-Sr-Ca-Cu-O system further contains Pb.
 5. A process accordingto claim 1, wherein said stack of films as deposited has an averagecomposition including a ratio of Bi/Pb of 1/0.5 to 1/1.5.
 6. A processaccording to claim 5, wherein said stack of films as deposited has anaverage composition including a ratio of Bi/Pb of 1/0.6 to 1/0.9.
 7. Aprocess according to claim 1, wherein said stack of films has an averagecomposition including a ratio of Sr/Cu of 1/1.5 to 1/1.7.